Communications of the ACM
Soft Robotics is About More than Building Robots
Time-lapse swimming trajectory of a soft robotic fish swimming in a coral reef in Fiji.
Credit: Robert Katzschmann
Some see soft robots helping declining populations of pollinators do their jobs, or sifting through wreckage in the wake of a building collapse, or even performing simple, practical tasks in tight spaces; others see them traveling the oceans or traversing the insides of our bodies to scope out medical red flags.
Some, like Robert Katzschmann, an assistant professor of robotics at ETH Zurich in Switzerland, see the need to tread a little more lightly and a lot more quietly in our world, which is one of the reasons he's building soft robots: to help us better integrate with nature.
A study by Research and Markets predicts the market for soft robotics will reach $2.16 billion by 2024 as the versatility of soft robots takes center stage. Metallic robots made of rigid metals and plastics, that use rotating motors or fast spin propellers, are designed and are constructed with speed and precision in mind; that makes them key drivers in industrial settings and in assembly line work. However, says Katzschmann, "There are no rotating motors in nature. Nature uses muscles to smoothly wiggle, walk or run. Muscles combined with soft materials make for very adaptive and safe environments that you can use in your everyday life."
Soft robots are made from materials that can approximate biological functions. In fact, the researchers in Katzschmann's lab—chemists, material scientists, biologists, physicists, computer scientists, data scientists, and roboticists—are finding ways to make machines from live, contracting muscles. "If you want to really have robots be ubiquitous, be among us, they have to be made physically of something that at least mechanically matches us," he says. If you can do that, he says, you can build a "future that's more sustainable, and [one] that's also preserving nature, without all this extra noise that comes from traditional machines."
Katzschmann's new generation of robots have silent muscles that directly transform electrical energy into contractions. "Imagine a submarine that has a normal propeller at its back, but it actually swims by moving its formable tail from side to side, just like a real fish," says Katzschmann, who is particularly concerned about the impact loud machine noise has on marine life. Whales, he says, can hardly communicate with each other over the noises from ships, ocean liners, boats, and submarines that contaminate the oceans.
"As we know, a human that is constantly exposed to noise can become depressed and even develop tinnitus. So now try to imagine living in a world without all this noise by machines. How would your day-to-day life look in a busy city, without these rattling noises? Instead of this noise, the world would be filled with natural and human noises primarily, and if you rethink the way of making transportation systems or even robotic automation systems, this could actually improve the sound of our worlds and at the same time, preserve nature."
A decade ago, Katzschmann found himself caught between roboticists, whom he says understand how to put together models they can control, and material scientists, who understand how to combine new materials. This led him to a realization: "If I use the right materials and put them into robots, I can actually have real muscles in the robots," he says.
Though still in the early stages, Katzschmann is taking cardiac cells and skeletal muscle cells from lab rats and mice, growing and even genetically modifying them so they will contract in reaction to light or electricity.
Katzschmann and the biologists in his lab are building cell lines that will proliferate forever (so he doesn't have to continue sacrificing animal lives). He's also looking at using cells from non-mammalian species, like crickets and other insects. "There's people who have shown that they can extract the muscle cell from an insect and they could grow some of these cells in a petri dish. And what we are interested in now is working together with people from Tufts (University) that they will get us the immortalized cell line of this muscle cell from a particular insect and then we would use it. So you see, there's these opportunities for thinking really outside of the box."
With the help of a hydrogel that can be three-dimensionally (3D) printed, the cells have a "cue" of where and how to grow, says Katzschmann, so they may be used to grow a small muscle of about one to two centimeters in a petri dish. By putting electrodes on its sides, you can make the muscle work, contracting and pulling left or right.
Katzschmann says the two greatest challenges his team now faces are "vascularization and good cell alignments." They are trying to figure out how to get the cells to align properly, and how to keep the cells alive more than a few days, which could require a micro-vascular system.
Fellow roboticist Xiaobo Tan, a Foundation Professor of electrical and computer engineering at Michigan State University, finds Katzschmann's work inspiring. "He's really working on cutting edge problems here. …He's got a bio hybrid actuator [that] I would characterize as kind of a cyborg device, but basically combining biological and computing/mechanical elements to create something functional … I think this is great stuff."
Katzschmann's eventual goal is to develop "truly cradle-to-cradle robots, so that you have, in the end, a robot that is actually sustainable in the sense that I have a robot that was grown, and once it's not needed anymore, I can just compost it, I can just put it back to the soil."
He acknowledges it is a bit ambitious to say he would have a muscle-powered robot built in the next 10 years, adding, "I can say in five to 10 years, we definitely will have robots that are made with artificial muscles from electrostatic principles."
The Robotic Bee
"The main objective [of soft robotics] is not to make super-precise machines, because we already have them," biomedical engineer Giada Gerboni said in a 2018 TED Talk, "but to make robots able to face unexpected situations in the real world."
In fact, that is one of the key components of the bee robot under construction by Kevin Chen. An assistant professor in the department of Electrical Engineering and Computer Science (EECS) at the Massachusetts Institute of Technology (MIT), Chen is building small-scale flying robots that are powered soft actuators that feel and operate like muscles (although unlike Katzschmann's, they are artificial).
With a background in physics, Chen says before even starting to develop a soft bee robot, he needed to understand the biomechanics and the physics and scope of the insect, including its aerodynamics; wing flap; how the wings lift and, in turn, lift the bee's body; how the bee's wings and body handle drag forces, and how bees interact with their environment.
Chen needed to build an artificial muscle because, unlike Katzschmann and his colleagues, Chen is not using live muscle cells. He started by building a dielectric elastomer actuator—a soft capacitor established by a soft elastomer made of thin rubber cylinders coated in carbon nanotubes. The robot's wings flutter when voltage is applied and an electrostatic force squeezes and elongates the rubber cylinder. "The elastomer layers are about 10 to 40 microns—still pretty thin, but the carbon nanotube layers are a nanometer in thickness, so very, very thin," explains Chen, who says his insect robots weigh only .6 grams, the approximate weight of a bumblebee.
He adds, "By looking at the underlying physics, what we can do is allow this contraction and elongation process to happen very, very quickly." As a result, the wings of his reobot bee flap nearly 500 times per second, faster than an insect's biological muscles.
According to Chen, applications for the bee robot run the gamut from pollination to rescue efforts. Chen's team wants its robots to have insect-like resilience, to sense collisions and rebound safely from them. Imagine, Chen suggests, how a large swarm of tiny bee robots could help find people trapped inside a collapsed building without triggering a secondary collapse, like a hard robot might from its weight or hard shell. The bee robot would be more robust in such a situation than a drone, and would be able to fit into incredibly small spaces. "The goal is not to pull people out. The goal is really to sense if there are survivors located in that area. I think that can be quite useful," says Chen.
Chen says he and his team are working on ways to improve his insect robot's accuracy and energy efficiency. He believes the research could yield a successful bee robot in the next five to 10 years.
Michigan State's Tan calls Chen's goals "very ambitious but important." Tan says Chen's project presents many challenges and, therefore, he expects that it will be more like 10 to 20 years before we'll see Chen's bees in the marketplace.
Tan notes that Chen "is advancing one of the most important pieces here, which is the actuators. I think that some of his recent work looks at how do you actually optimize some construction of these actuators to increase the power density, to lower the required voltage," says Tan.
"The technologies that he is trying to advance will not only be really useful for this bee robot project, but also other soft robots … like manipulators or other types of mobile robots, flying, swimming, climbing, crawling. I can see that use of advancement in what he's working on here."
Cari Shane is a journalist based in the Washington D.C. area, a generalist with a keen interest in health, medicine, and science writing.
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